refactor: make country and state associations optional

- Change 'belongs_to :country' association to be optional
- Change 'belongs_to :state' association to be optional

This refactoring allows a City record to exist without having
an associated Country or State, thus increasing the flexibility
of the data model for various use cases.

app/models/city.rb

@@ -1,7 +1,7 @@
 class City < ApplicationRecord
   extend FriendlyId
   friendly_id :slug_candidates, use: :slugged
-  belongs_to :country
+  belongs_to :country, optional: true
   belongs_to :state, optional: true
 
   has_many :weather_arts, dependent: :destroy